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Cometary nuclei consist mostly of dust and water ice (1). Previous observations have found nuclei to be low-density and highly porous bodies (2-4), but have only moderately constrained the range [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.09.001 Byline: F. Postberg (a), S. Kempf (a)(b), J.K. Hillier (c), R. Srama (a), S.F. Green (c), N. McBride (c), E. Grun (a)(d) Keywords: Saturn; Enceladus; Interplanetary dust; Planetary rings Abstract: The population of Saturn's outermost tenuous E-ring is dominated by tiny water ice particles. Active volcanism on the moon Enceladus, embedded in the E-ring, has since late 2005 been known to be a major source of particles replenishing the ring. Therefore particles in the vicinity of Enceladus may provide crucial information about the dynamical and chemical processes occurring below the moon's icy surface. Here we present a statistical evaluation of more than 2000 impact ionisation mass spectra of Saturn's E-ring particles, with sizes predominantly below 1 [mu]m, detected by the Cosmic Dust Analyser onboard the Cassini spacecraft. We focus on the identification of non-water features in spectra otherwise dominated by water ice signatures. Here we specify the categorisation of two different spectrum types, which probably represent two particle populations. Type I spectra imply pure water ice particles, whereas in Type II spectra organic compounds and/or silicate minerals are identified as impurities within the icy particles. This finding supports the hypothesis of a dynamic interaction of Enceladus' rocky core with liquid water. Author Affiliation: (a) Max-Planck-Institut fur Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany (b) Institut fur Geophysik und Extraterrestrische Physik, Universitat Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany (c) Planetary and Space Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK (d) Hawaii Institute of Geophysics and Planetology, University of Hawaii, 1680 East West Road, Honolulu, HI 96822, USA Article History: Received 8 January 2007; Revised 3 September 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.06.027 Byline: S. Kempf (a)(b), U. Beckmann (a), G. Moragas-Klostermeyer (a), F. Postberg (a), R. Srama (a), T. Economou (c), J. Schmidt (d), F. Spahn (d), E. Grun (a)(e) Keywords: Saturn; rings; Saturn; satellites; Volcanism; Impact processes Abstract: Saturn's diffuse E ring is the largest ring of the Solar System and extends from about 3.1R.sub.S (Saturn radius R.sub.S=60,330km) to at least 8R.sub.S encompassing the icy moons Mimas, Enceladus, Tethys, Dione, and Rhea. After Cassini's insertion into her saturnian orbit in July 2004, the spacecraft performed a number of equatorial as well as steep traversals through the E ring inside the orbit of the icy moon Dione. Here, we report about dust impact data we obtained during 2 shallow and 6 steep crossings of the orbit of the dominant ring source -- the ice moon Enceladus. Based on impact data of grains exceeding 0.9 [mu]m we conclude that Enceladus feeds a torus populated by grains of at least this size along its orbit. The vertical ring structure at 3.95R.sub.S agrees well with a Gaussian with a full-width-half-maximum (FWHM) of [approximately equal to]4200km. We show that the FWHM at 3.95R.sub.S is due to three-body interactions of dust grains ejected by Enceladus' recently discovered ice volcanoes with the moon during their first orbit. We find that particles with initial speeds between 225 and 235 mas.sup.-1 relative to the moon's surface dominate the vertical distribution of dust. Particles with initial velocities exceeding the moon's escape speed of 207 mas.sup.-1 but slower than 225 mas.sup.-1 re-collide with Enceladus and do not contribute to the ring particle population. We find the peak number density to range between 16x10.sup.-2m.sup.-3 and 21x10.sup.-2m.sup.-3 for grains larger 0.9 [mu]m, and 2.1x10.sup.-2m.sup.-3 and 7.6x10.sup.-2m.sup.-3 for grains larger than 1.6 [mu]m. Our data imply that the densest point is displaced outwards by at least 0.05R.sub.S with respect of the Enceladus orbit. This finding provides direct evidence for plume particles dragged outwards by the ambient plasma. The differential size distribution n(s.sub.d)ds.sub.d[approximately equal to] s.sub.d.sup.-q.sub.sds.sub.d for grains 0.9[mu]m is described best by a power law with slopes between 4 and 5. We also obtained dust data during ring plane crossings in the vicinity of the orbits of Mimas and Tethys. The vertical distribution of grains 0.8[mu]m at Mimas orbit is also well described by Gaussian with a FWHM of [approximately equal to]5400km and displaced southwards by [approximately equal to]1200km with respect to the geometrical equator. The vertical distribution of ring particles in the vicinity of Tethys, however, does not match a Gaussian. We use the FWHM values obtained from the vertical crossings to establish a 2-dimensional model for the ring particle distribution which matches our observations during vertical and equatorial traversals through the E ring. Author Affiliation: (a) MPI fur Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany (b) IGEP, Universitat Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany (c) Laboratory for Astrophysics and Space Research, University of Chicago, Chicago, IL 60637, USA (d) Institut fur Physik, Universitat Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany (e) Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, USA Article History: Received 13 January 2007; Revised 22 June 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.02.018 Byline: M.J. Willis (a), M.J. Burchell (b), T.J. Ahrens (a), H. Kruger (c), E. Grun (d)(e) Keywords: Interplanetary dust; Impact processes; Instrumentation; Experimental techniques Abstract: Experiments to investigate the effect of impacts on side-walls of dust detectors such as the present NASA/ESA Galileo/Ulysses instrument are reported. Side walls constitute 27% of the internal area of these instruments, and increase field of view from 140[degrees] to 180[degrees]. Impact of cosmic dust particles onto Galileo/Ulysses Al side walls was simulated by firing Fe particles, 0.5-5 [mu]m diameter, 2-50 kmas.sup.-1, onto an Al plate, simulating the targets of Galileo and Ulysses dust instruments. Since side wall impacts affect the rise time of the target ionization signal, the degree to which particle fluxes are overestimated varies with velocity. Side-wall impacts at particle velocities of 2-20 kmas.sup.-1 yield rise times 10-30% longer than for direct impacts, so that derived impact velocity is reduced by a factor of [approximately equal to]2. Impacts on side wall at 20-50 kmas.sup.-1 reduced rise times by a factor of [approximately equal to]10 relative to direct impact data. This would result in serious overestimates of flux of particles intersecting the dust instrument at velocities of 20-50 kmas.sup.-1. Taking into account differences in laboratory calibration geometry we obtain the following percentages for previous overestimates of incident particle number density values from the Galileo instrument [Grun et al., 1992. The Galileo dust detector. Space Sci. Rev. 60, 317-340]: 55% for 2 kmas.sup.-1 impacts, 27% at 10 kmas.sup.-1 and 400% at 70 kmas.sup.-1. We predict that individual particle masses are overestimated by [approximately equal to]10-90% when side-wall impacts occur at 2-20 kmas.sup.-1, and underestimated by [approximately equal to]10-10.sup.2 at 20-50 kmas.sup.-1. We predict that wall impacts at 20-50 kmas.sup.-1 can be identified in Galileo instrument data on account of their unusually short target rise times. The side-wall calibration is used to obtain new revised values [Kruger et al., 2000. A dust cloud of Ganymede maintained by hypervelocity impacts of interplanetary micrometeoroids. Planet. Space Sci. 48, 1457-1471; 2003. Impact-generated dust clouds surrounding the Galilean moons. Icarus 164, 170-187] of the Galilean satellite dust number densities of 9.4x10.sup.-5, 9.9x10.sup.-5, 4.1x10.sup.-5, and 6.8x10.sup.-5m.sup.-3 at 1 satellite radius from Io, Europa, Ganymede, and Callisto, respectively. Additionally, interplanetary particle number densities detected by the Galileo mission are found to be 1.6x10.sup.-4, 7.9x10.sup.-4, 3.2x10.sup.-5, 3.2x10.sup.-5, and 7.9x10.sup.-4m.sup.-3 at heliocentric distances of 0.7, 1, 2, 3, and 5 AU, respectively. Work by Burchell et al. [1999b. Acceleration of conducting polymer-coated latex particles as projectiles in hypervelocity impact experiments. J. Phys. D: Appl. Phys. 32, 1719-1728] suggests that low-density 'fluffy' particles encountered by Ulysses will not significantly affect our results -- further calibration would be useful to confirm this. Author Affiliation: (a) Lindhurst Laboratory of Experimental Geophysics, Seismological Laboratory, 252-21, 1200 E. California Blvd., California Institute of Technology, Pasadena, CA 91125, USA (b) Centre for Astrophysics & Planetary Sciences, School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NR, UK (c) Max-Planck-Institut fur Sonnensystemforschung, Max-Planck-StraAe 2, 37191 Katlenberg-Lindau, Germany (d) Max-Planck-Institut fur Kernphysik, Postfach 103980, 69029 Heidelberg, Germany (e) Hawaii Institute of Geophysics and Planetology, University of Hawaii, 1680 East West Road, Honolulu, HI 96822, USA Article History: Received 20 May 2004; Revised 17 January 2005

We present the characteristics of the dust comae of two comets, 126P/IRAS, a member of the Halley family (a near-isotropic comet), and 2P/Encke, an ecliptic comet. We have primarily used mid- and far-infrared data obtained by the ISOPHOT instrument aboard the Infrared Space Observatory (ISO) in 1996 and 1997, and mid-infrared data obtained by the SPIRIT III instrument aboard the Midcourse Space Experiment (MSX) in 1996. We find that the dust grains emitted by the two comets have markedly different thermal and physical properties. P/IRAS's dust grain size distribution appears to be similar to that of fellow family member 1P/Halley, with grains smaller than 5 microns dominating by surface area, whereas P/Encke emits a much higher fraction of big (20 [micro]m and higher) grains, with the grain mass distribution being similar to that which is inferred for the interplanetary dust population. P/Encke's dearth of micron-scale grains accounts for its visible-wavelength classification as a 'gassy' comet. These conclusions are based on analyses of both imaging and spectrophotometry of the two comets; this combination provides a powerful way to constrain cometary dust properties. Specifically, P/IRAS was observed preperihelion while 1.71 AU from the Sun, and seen to have a 15-arcmin long mid-infrared dust tail pointing in the antisolar direction. No sunward spike was seen despite the vantage point being nearly in the comet's orbital plane. The tail's total mass at the time was about 8 x [10.sup.9] kg. The spectral energy distribution (SED) is best fit by a modified greybody with temperature T = 265 [+ or -] 15 K and emissivity [epsilon] proportional to a steep power law in wavelength [lambda] : [epsilon] [varies] [[lambda].sup.-[alpha]] where [alpha] = 0.50 [+ or -] 0.20 (2 [sigma]). This temperature is elevated with respect to the expected equilibrium temperature for this heliocentric distance. The dust mass loss rate was between 150-600 kg/s (95% confidence), the dust-to-gas mass loss ratio was about 3.3, and the albedo of the dust was 0.15 [+ or -] 0.03. Carbonaceous material is depleted in the comet's dust by a factor of 2-3, paralleling the [C.sub.2] depletion in P/IRAS's gas coma. P/Encke, on the other hand, observed while 1.17 AU from the Sun, had an SED that is best fit by a Planck function with T = 270 [+ or -] 15 K and no emissivity falloff. The dust mass loss rate was 70-280 kg/s (95% confidence), the dust-to-gas mass loss ratio was about 2.3, and the albedo of the dust was about 0.06 [+ or -] 0.02. These conclusions are consistent with the strongly curved dust tail and bright dust trail seen by Reach et al. (2000; Icarus 148, 80) in their ISO 12-[micro]m imaging of P/Encke. The observed differences in the P/IRAS and P/Encke dust are most likely due to the less evolved and insolated state of the P/IRAS nuclear surface. If the dust emission behavior of P/Encke is typical of other ecliptic comets, then comets are the major supplier of the interplanetary dust cloud. Keywords: Comets; Infrared observations; Photometry; Dust

Dust particles in interplanetary space are expected to charge up to an electrostatic potential of about +5 V mostly by the solar UV (Horanyi, 1996, Annu. Rev. Astrophys. 34, 383). Since the dynamics of charged grains may be quite different from neutral particles, the knowledge of the grain charge [Q.sub.d] is highly desirable. In the last two decades, several detectors on spacecraft were flown to measure the dust charge in-situ, but the instrumentation was not capable of determining the dust charge unambiguously. The Cosmic Dust Analyser (CDA) on the Cassini spacecraft includes a charge sensitive entrance grid system (QP detector). While entering the instrument, sufficiently charged particles induce a characteristic charge feature onto the grid system, which allows a reliable determination of [Q.sub.d] as well as of the impact speed [[upsilon].sub.d]. Here we report the first successful in-situ measurement of charged interplanetary dust grains by CDA. Amongst 37 impacts by interplanetary grains registered between November 1999 and January 2000, we identified 6 impacts whose QP signals show a clear feature caused by charged grains, corresponding to [Q.sub.d] between 1.3 and 5.4 fC. Knowledge of [Q.sub.d] also allows us to estimate the grain mass [m.sub.d]. Assuming a potential of [[phi].sub.d] [approximately equal to] +5 V and spheroidal grain morphologies with ratios of the maximum size to the minimum size of less than 2 the masses derived from [Q.sub.d] were found to be in excess of [10.sup.-13] kg. The dynamics of such particles are dominated by the Sun's gravity. In the framework of the micro-meteoroid models of the Solar System these grains belong to the core population of interplanetary grains (Divine, 1993, J. Geophys. Res. 98, 17029). Furthermore, a rate of 6 impacts of grains with [m.sub.d] [greater than or equal to] [10.sup.-13] kg during 107 days is in good agreement with the predictions of the interplanetary dust environment model by Staubach et al. (1997, Adv. Space Res. 19, 301). This result demonstrates that charge detectors as the CDA QP system offer a reliable in-situ technique for determining simultaneously both the mass and velocity of big interplanetary grains. The primary CDA subsystem to determine [m.sub.d] and [[upsilon].sub.d], however, is an impact ionisation detector. The majority of the 37 recorded dust impacts produced impact charges are well outside the calibrated range. Moreover, these impacts were usually characterised by impact ionisation signals which differ significantly from signals taken in calibration experiments. In this paper we took advantage of the fact that the measurement of [Q.sub.d] is not affected by the subsequent impact of the grain with the detector. By relating [m.sub.d] and [[upsilon].sub.d] derived from [Q.sub.d] of the 6 QP impactors to their corresponding ionisation signals we show that in many cases even for energetic impacts outside the calibrated range meaningful values for the dust mass can be obtained. The observed deviations of the ionisation signals from the calibration measurements are likely due to the large amount of plasma generated by such impacts. We discuss the implications of these findings on a meaningful reduction of impact ionisation signals caused by big particle impacts. A new scheme to identify and to evaluate such signals is presented. These finding are of great importance for future Cassini measurements in the saturnian system. Keywords: Interplanetary dust: Solar wind; Celestial mechanics: Data reduction techniques

Identical in situ dust detectors are flown on board the Galileo and Ulysses spacecraft. They record impacts of micrometeoroids in the ecliptic plane at heliocentric distances from 0.7 to 5.4 AU and in a plane almost perpendicular to the ecliptic from -79 [degrees] to +79 [degrees] ecliptic latitude. The combination of both Ulysses and Galileo measurements yields information about the radial and latitudinal distributions of micron- and submicron-sized dust in the Solar System. Two types of dust particles were found to dominate the dust flux in interplanetary space. Interplanetary micrometeoroids covering a wide mass range from [10.sup.-16] to [10.sup.-6] g are recorded mostly inside 3 AU and at latitudes below 30 [degrees]. Interstellar grains with masses between [10.sup.-14] and [10.sup.-12] g have been positively identified outside 3 AU near the ecliptic plane and outside 1.8 AU at high ecliptic latitudes (>50 [degrees]). Interstellar grains move on hyperbolic trajectories through the planetary system and constitute the dominant dust flux (1.5 x [10.sup.-4] [m.sup.-2] [sec.sup.-1]) in the outer Solar System and at high ecliptic latitudes. To compare and analyze the Galileo and Ulysses data sets, a new model is developed based on J. Geophys. Res. 98, 17029-17048, Divine's (1993, 'five populations of interplanetary meteoroids' model. Both models describe the interplanetary meteoroid environment in terms of dust populations on distinct orbits. Taking into account the measured velocities and the effect of radiation pressure on small particles (described by the ratio of radiation pressure force to gravity, [Beta]), we define four populations of meteoroids on elliptical orbits and one population on hyperbolic orbit that can fit the micrometeoroid flux observed by Galileo and Ulysses. Micrometeoroids with masses greater than [10.sup.-10] g and negligible radiation pressure ([Beta] = 0) orbit the Sun on low to moderately eccentric orbits and with low inclinations ([less than or equal to] 30 [degrees]). Populations of smaller particles with mean masses of [10.sup.-11] g ([Beta] = 0.3), [10.sup.-13] g ([Beta] = 0.8), and 5 x [10.sup.-15] g ([Beta] = 0.3), respectively, have components with high eccentricities and have increasingly wider inclination distributions with decreasing mass. Similarities among the orbit distributions of the small particle populations on bound orbits suggest that all are genetically related and are part of an overall micrometeoroid complex that prevails in the inner Solar System. The high-eccentricity component of the small particle populations may actually be [Beta]-meteoroids which are not well characterized by our measurements. Our modeling suggests further that the interstellar dust flux is not reduced at Ulysses' perihelion distance (1.3 AU) and that it contributes about 30% of the total dust flux observed there.

The dust in the dust streams detected by the Galileo spacecraft near Jupiter originate from either the jovian gossamer ring or volcanoes on Io. The dust does not originate from the Shoemaker-Levy 9 comet as the comet has no stream activity. The dust storms detected by the Galileo dust detector are more intense and longer than those observed by the Ulysses spacecraft. The dust stream activity changes with time and there are significant differences between the intensity of the strongest and the weakest streams.

Comet P/Faye 1991 XXI (1991 n) was observed with the planetary camera (PC) of the Hubble Space Telescope (HST) between October 29 and November 21, 1991, when its geocentric distance was in the range 0.62-0.67 AU. The resulting high resolution - a single PC pixel projected to a distance of [appoximately]20 km at the comet - made it possible to clearly discriminate the nucleus and to study the dust coma within [approximately]100 km from the nucleus. The spherical aberration which affected the early operation of the HST has severely complicated the analysis and image restoration using the Richardson-Lucy method has failed to give satisfactory results. The coma is dominated by the point spread function (PSF) of the nucleus up to a radial distance of [approximately]100 km and cannot be recovered. Beyond, it is affected by the spherical aberration up to approximately 750 km and was analyzed by comparison with a grid of models convolved with appropriate PSFs. Beyond 750 km, it remains unaffected and can be studied directly. From the outer to the innermost regions, the coma presents an elongated shape which may be explained either by an active source on the nucleus or, more likely, by a projection effect of the dust tail. If this second interpretation is correct, then the temporal evolution of the comet is very slow and smooth, suggesting an extended source of dust on the nucleus. The dust production rate, corrected for the projection effect of the tail, amounts to 125 kg [sec.sup.-1].

Along Ulysses' path from Jupiter to the south ecliptic pole, the onboard dust detector measured a dust impact rate that varied slowly from 0.2 to 0.5 impacts per day. The dominant component of the dust flux arrived from an ecliptic latitude and longitude of 10' ± 1O° and 280° [+ or - 30] ° which indicates an interstellar origin. An additional flux of small particles, which do not come from the interstellar direction and are unlikely to be zodiacal dust grains, appeared south of -45° latitude. One explanation is that these particles are beta-meteoroids accelerated away from the sun by radiation pressure and electromagnetic forces., The objective of the Ulysses dust detector is to measure impact directions, velocities, and masses of dust in the solar system. Here we report on results from the orbital arc [...]

Images of Saturn's B ring were used to analyze the spoke phenomenon. The position of spoke edges was measured for 150 different spokes on up to five subsequent images each. Spoke edges move in a manner similar to Keplerian orbital velocity. A rough model of the spoke visibility was constructed as a function of the spoke age and this was fitted to the observations. Results indicate an active time for spoke visibility increase, that spoke decay takes place faster than spoke growth does and that trailing edges vary little during the lifetime of a spoke.

Author(s): A. L. Graps (corresponding author) [1]; E. Grün [1]; H. Svedhem [2]; H. Krüger [1]; M. Horányi [3]; A. Heck [1]; S. Lammers [1] Streams of dust emerging from [...]

A model of the dust acceleration and ejection in Jupiter's magnetosphere has yielded mass and velocity ranges consistent with the findings of the Ulysses mission. The dust events observed by the said mission resulted from the actions of the jovial magnetospheree on dust particles. The model also showed that only particles in a given size range can acquire enough energy to be ejected from Jupiter's gravitational influence.

The results of the European Space Agency's Giotto Dust Impact Detection System are reported. The Giotto spacecraft came within 200 km of the nucleus of the comet Grigg-Skjellerup. Its front shield registered impacts from three meteoroid with sizes of 100, 2 and 20 micro-grams, indicating that large particles predominate in the cometary dust. The data is consistent with that from a 1986 encounter with the comet Halley, indicating a higher rate of mass loss, and thus a higher dust-to-gas ratio, from the nucleus of comets.

Two types of dust particles were detected by the Ulysses spacecraft during its passage near Jupiter last Feb 8, 1992. The first type occured at periodic bursts and measured in the submicrometer range. The streams were narrow, collimated and concentrated near Jupiter. These grains are hypothesized to be of Jovian origin, which escaped Jupiter's influence by a combined action of the plantet's intense magnetic field and the high solar wind speed. The second type of dust particles were interstellar dust. This was indicated by the retrograde motion of the particles which moved at speeds comparable to interstellar gas.